The oven is built out of 55 gallon drums and can be made inexpensively.
The oven can be built in different ways. One method uses two drums. The inner drum holds the bread. It is completely closed except for three doors cut into the front of the barrel. Six layers of aluminum foil, each separated by a small air space, insulate between the two barrels. (We've also built bread ovens without aluminum foil. Instead a third barrel surrounds the second. Insulation fills a 4" gap between the 2nd and 3rd barrels.) A gap of about an inch, between the inner barrel and the foil, allows the hot flue gases to rise up between the two barrels. In effect this gap becomes a part of the chimney.
Many elements act together to make this an effi-
cient oven:
The hot air is in contact with the bottom, the sides, and the top of the inner drum which contains the bread.
The hot air is forced through a tight gap that forces the heat to touch the inner drum, heating it more efficiently than would happen if the gap were wider.
The inner drum is low mass and is insulated by the aluminum foil that creates air gaps between each layer. (Aluminum foil also
reflects infrared radiation back into the inner drum, at the bread.)
The insulated firebox keeps the fire hot, assisting complete combustion and reducing smoke.
The insulated chimney creates a great deal of draft and air is preheated before it can cool the fire.
The fuel is either fed vertically or horizontally. Vertical feed allows for more preheating of air and the wood is more self-feeding. Horizontal feed is more what people are used to, and
The downfeed-downdraft bread oven.
doesn't suffer from backdrafting as much. Horizontal feed is only slightly less efficient.
The fire chamber and fuel magazine can be formed in various ways. (Please refer to the drawings.) The oven at the Research Center uses a 14 gallon drum as the feed box. It has a fired clay liner inside it, surrounded by pumice rock as the insulation. This liner creates the fire chamber and tunnel that directs the fire at the bottom of the inner drum. (It's also possible to make a horizontal fire box patterned after the smaller Rocket cooking stove.)
A metal cap, also insulated, covers both barrels and sits 1" above the inner drum, allowing the flue gases to reach the chimney unimpeded. A six inch in diameter section of chimney pipe exits vertically from the cap.
Three shelves fit into the rings of the barrel. We cut circular shelves out of metal fencing to be a bit bigger than the diameter of the drum. These shelves were forced down into the barrel, to spring flat and find support in the rings.
The doors were cut as cleanly as possible. A large door, covering all three holes, swings on hinges attached to the outer barrel. It closes against two pieces of angle iron that fill the gap between the two barrels.
This oven saves a great deal of fuel when compared to a beehive earthen oven. People cannot believe that a few sticks can be heating the entire interior of the drum to cooking temperatures.
In Mexico, it's been the design most universally praised. It's easy, with a bit of practice, to regulate the temperature, and large amounts of food can be made at one time. We recommend it very highly!
Earthen stoves can use so much wood, to bake so little bread! At the very least, earthen stoves can be covered by insulation (wood ash, etc.) so that the heat isn't as easily lost. The low mass oven heats to 350 degrees F. in twenty minutes and keeps at that temperature with small branches added to the fire.
The split drum slides over the inner barrel. How flue gases pass between the drums.
thanks for your suggestion of chimney pipe.
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